The invention relates to a recirculation structure for turbocompressors as well as to an aircraft engine and an industrial gas turbine having a ring chamber arranged concentrically with respect to compressor axis in an area of free blade ends of a blade ring, an axial center of the ring chamber being situated upstream of an axial center of the free blade ends, and the ring chamber radially adjoining a contour of a mian flow duct annulus, and having a plurality of guiding elements which are arranged in the ring chamber, are distributed along its circumference and are arranged and shaped such that in an axially trailing area of the ring chamber, entry of recirculation flow takes place in an advantageous manner with respect to the flow, and in an axially leading area of the ring chamber, exit of the recirculation flow relative to the downstream blade ring takes place in a defined direction and optionally with a define swirl, the guiding elements having recesses for the flow passage in the circumferential direction, in the leading and/or trailing areas of the ring chamber.
Recirculation structures for turbocompressors have been known for some time and, as a rule, are called “casing treatments” in the art. These primarily have the task of increasing the aerodynamically stable operating range of the compressor, in which case the so-called surging limit is displaced toward higher compressor pressures, that is, toward a higher compressor stress. The disturbances responsible for a local stall and, in the end, for the surging of the compressor, on the casing side, occur at the moving blade ends of one or more compressor stages and, on the hub side, at the radially interior guide blade ends because, in these areas, the aerodynamic stress is the highest. As the result of the recirculation of the “air particles” circulating between the blade tips at the rotational blade speed and having reduced energy into the main stream while the energy is increased, the flow in the area of blade ends is stabilized again. Since, as a rule, flow disturbances do not occur uniformly over the stage range, it should be possible to fluidically, achieve a compensation in the circumferential direction in addition to the essentially axial recirculation. The main disadvantage of the known “casing treatments” is the fact that, although they increase the surging limit, they equilaterally reduce the pressure coefficient.
German Patent Document DE 33 22 295 C3 protects an axial fan having a “casing treatment” of the above-mentioned type. A ring chamber (8) is illustrated there in which guiding elements (9) are fixedly arranged. In the downstream area over the running blade ends, an area is situated which is open in the circumferential direction and into which the guiding elements do not extend. A closed ring (7), which is aligned approximately with the contour of the main flow duct, is characteristic of this type of “casing treatment”, which ring (7) separates the trailing entry area from the leading outlet area of the recirculation structure and forms a smooth closed surface area.
A very similar “casing treatment” is known from German Patent Document DE 35 39 604 C1, in which case, an area exists in the leading and trailing area of the ring chamber (7) which is open in the circumferential direction. The radially interior ring 6 should also be observed here.
A more novel “casing treatment” is known from U.S. Patent Document U.S. Pat. No. 5,282,718 A. Here, the ring chamber (18, 28) and the guiding elements (24) are fluidically refined. Here also, the inlet and the outlet of the recirculation flow are separated by a solid ring which is smooth and closed toward the blades. Such rings in the blade area, as a rule, have to be equipped with a grazing or abradable coating in the event of a contact with the blade tips.
Additional “casing treatments” with axial or axially diagonal grooves, are disclosed, for example, in U.S. Patent Document U.S. Pat. No. 5,137,419 A. These are not considered here because, in the absence of a mutual connection of the grooves, no flow compensation can take place in the circumferential direction in these versions.
In view of the disadvantages of the solutions according to the prior art, it is an object of the invention to provide a recirculation structure for turbocompressors, which permits clear increase of the surging limit and thus a clear enlargement of the stable operating range without any relevant impairment of the pressure coefficient.
The essence of the invention is the fact that the ring chamber with the guiding elements is completely open toward the main flow duct along its axial length and its circumference. Ring-type elements with grazing coatings, etc. are eliminated. The above-mentioned patent documents demonstrate that, up to now, the technical world had attempted to construct recirculation structures toward the main flow duct, that is, toward the so-called annulus, to be smooth, without gaps and closed along an axial area which is as large as possible in order to cause a lengthening of the contour of the main flow duct which is as favorable to the flow as possible and has low losses. In contrast, the invention results in gaps, fissured surfaces, etc. and therefore appears to be disadvantageous and in expedient. However, tests have shown that the recirculation structure according to the invention is superior to known solutions with respect to the rise of the surging limit as well as with respect to the efficiency. Aerodynamically, this can be explained in that the free and unforced forming of the recirculation flow in the open ring chamber with free standing guiding elements and flow connections in the circumferential direction is more important than a lengthening of the contour of the main flow duct which is as gap-free as possible. The absence of a closed ring has the additional advantages that no grazing or abradable (inlet) coating is required for the guiding elements, and radial space as well as weight are saved which results in structure-mechanical advantages.
Preferred embodiments of the recirculation structure according to the main claim are characterized in the subclaims.
The invention will be explained in detail in the following by means of the drawing.